The invention relates to a method for noncontact, radiation thermometric temperature measurement. According to the method, photodiode radiation detector operating photovoltaically without bias voltage produces a short-circuit photocurrent which is proportional to the received radiant power. This photocurrent is processed in a current to voltage converter whereupon a temperature signal is generated which corresponds to the radiant power and, for example, is sent on to a temperature display. Moreover, the invention concerns a device for performing this method.
Noncontact, radiation thermometric temperature measuring methods as well as suitable devices, referred to as pyrometers, are known. The employed photodiode radiation detectors generate a photocurrent which is proportional to the radiant power. The radiant power of an object to be measured for a measuring wavelength of ≤2.4 μm can change, for example, in a temperature range of the object to be measured of 50° C. to 700° C., by almost six orders of magnitude. Correspondingly, the photocurrent changes also by six orders of magnitude.
In order to realize a great current dynamics range of approximately six decades in a continuous measuring range, different concepts are known. In the known methods, the photodiode current is converted to voltage which is subsequently further processed, for example, amplified.
The photodiode radiation detectors which are suitable for noncontact, radiation thermometric temperature measurement exhibit, depending on the material, different dark resistances or shunt resistances. This resistance amounts to several GΩ in case of silicon diodes with long wave sensitivity limit of λc=1.1 μm. In case of InGaAs diodes with λc=1.7 μm, the resistance amounts to several 10 MΩ. In case of extended InGaAs diodes with λc=2.6 μm, the resistance amounts to a few kΩ. Moreover, these shunt resistances are strongly dependent on the radiation detector temperature. In case of extended InGaAs photodiodes with λc=2.6 μm, the resistance decreases, for example, to one tenth for each temperature increase of 34K.
For the current to voltage conversion, operational amplifiers are employed. They have an input offset voltage and an input bias current. High-quality chopper operational amplifiers have a typical input offset voltage of ≤2.5 μV and an input bias current of ≤400 pA.
When measuring an object with low object temperature <75° C., the radiation intensity and thus also the photocurrent are very small. The photocurrent is within an order of magnitude of a few pA. Depending on the quality of the employed operational amplifier, the measurement of small photocurrents is error-prone to a great extent or such a measurement is even impossible. Methods for noncontact, radiation thermometric temperature measurement as well as the associated devices with photoelectric radiation detectors with a spectral range up to approximately 2.6 μm are therefore used only at temperatures above 100° C. Moreover, the object temperature should be approximately 25 to 30 Kelvin above the device temperature. For improving the methods and devices, temperature-stabilized radiation detectors can be employed which however require 15 to 30 minutes for heating up to operating temperature and also have an increased current consumption for heating. Other devices employ an optical chopper for signal stabilization according to the pulsating light method.
Object of the present invention is to improve a method for noncontact, radiation thermometric temperature measurement as well as a device suitable for the method in such a way that by means of a robust and potentially mobile device a simple and quick temperature measurement at object temperatures of <75° C., in particular at an object temperature at 50° C. and up, can be carried out accurately enough.